Processes Affecting the Benthic Flux of Trace Metals into the Water Column
of San Francisco Bay

ABSTRACT

Locally regulated point sources represent a significant input of trace metals
to San Francisco Bay, especially in the southern component (South Bay) during
the dry seasons and periods of drought. However, with recent recognition of
the importance of non-point sources, a variety of approaches are being used
to evaluate sediment remobilization and subsequent benthic flux of trace contaminants.
The extent to which chemical processes couple with diffusive and advective physical
processes in regulating benthic flux is also being examined. In terms of metal-speciation
controls on solute remobilization and benthic flux, we are investigating the
importance of metal-sulfide complexation relative to complexation with dissolved
organic substances by determining both metal and ligand fluxes simultaneously.
Consistent with previously reported water-column measurements, dissolved pore-water
sulfide concentrations, measured at two South Bay sites in July, 1997 and September,
1998, increased with sediment depth from around 10 nM near the sediment-water
interface to as high as 980 nM within the top 10 cm, suggesting a source to
the water column. Diffusive flux for sulfides estimated from pore-water profiles
ranged from 11 to 45 nmoles-m-2-h-1. As a complement to the pore-water approach,
core-incubation experiments were used to directly measure sulfide benthic flux
which ranged from 92 to 480 nmoles-m-2-h-1 over the same period. The significant
difference between these two approaches was attributable to biologically enhanced
advection (bioturbation/irrigation). Although sulfide benthic flux has been
found to be consistently positive (that is, out of the sediment as much as 948
nmoles-m-2-h-1), benthic fluxes for dissolved organic carbon (0.2 µm filtered,
DOC), copper and cadmium are temporally variable in direction across the sediment-water
interface, suggesting the importance of DOC complexation in regulating metal
benthic flux. Quantifying and understanding processes that affect the variability
of these fluxes would enhance water-quality modeling for this estuary.